JP3788350B2 - Exhaust gas purification device for internal combustion engine - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an exhaust gas purification apparatus for an internal combustion engine, and more particularly, traps NOx in exhaust when the air-fuel ratio of inflowing exhaust is lean, and reduces and purifies NOx trapped when the air-fuel ratio of inflowing exhaust is rich. It is related with what is provided with the NOx trap catalyst.
[0002]
[Prior art]
As a conventional technique, Japanese Patent No. 2600492 discloses trapping NOx in exhaust when the air-fuel ratio of inflowing exhaust is lean and trapping when the air-fuel ratio of inflowing exhaust is rich in the exhaust passage of the engine. A technology is disclosed in which a NOx trap catalyst for reducing and purifying NOx is disposed, and the exhaust air-fuel ratio is made rich at the reduction and purification timing of the NOx trap catalyst to purify NOx.
[0003]
[Problems to be solved by the invention]
In the above prior art, the reduction and purification of NOx by the NOx trap catalyst enriches the exhaust air-fuel ratio, that is, supplies HC and CO as reducing agents to the NOx trap catalyst, and these and NOx are in a reducing atmosphere. By reacting with
[0004]
However, oxygen may be present in the exhaust gas even when the exhaust air-fuel ratio is rich. If oxygen is present in the exhaust, the reaction between NOx and the reducing agent (HC, CO) in the reducing atmosphere will occur unless oxygen is consumed by the oxidation reaction of HC and CO to create a reducing atmosphere near the catalyst. Absent. For this reason, even in the same rich state (the same exhaust air-fuel ratio), the greater the amount of oxygen in the exhaust, the less the reduction reaction between the reducing agent and NOx occurs.
[0005]
That is, when the amount of oxygen in the exhaust gas per unit time flowing into the NOx trap catalyst is large, the ratio of the reaction between the reducing agent and oxygen increases in the NOx trap catalyst, and the reaction between the reducing agent and NOx is almost performed. Therefore, the NOx purification rate per unit time of the NOx trap catalyst decreases.
As described above, the conventional technique does not consider the presence of oxygen in the exhaust gas in the rich state in which NOx is reduced and purified. Therefore, when the amount of oxygen in the exhaust gas per unit time flowing into the NOx trap catalyst is large, There was a problem that NOx could not be sufficiently purified and exhaust was deteriorated.
[0006]
An object of the present invention is to provide an exhaust emission control device for an internal combustion engine that can solve such a conventional problem.
[0007]
[Means for Solving the Problems]
For this reason, according to the first aspect of the present invention, the NOx in the exhaust gas is trapped when the air-fuel ratio of the exhaust gas flowing into the engine is lean, and trapped when the air-fuel ratio of the exhaust gas flowing in is rich. A NOx trap catalyst for reducing and purifying NOx, a reduction and purification timing determination means for determining a reduction and purification timing of the NOx trap catalyst, a first air-fuel ratio enrichment method for enriching the exhaust air-fuel ratio at the reduction and purification timing, It is possible to selectively switch between the second air-fuel ratio enrichment method in which the exhaust air-fuel ratio is made the same value as in the first air-fuel ratio enrichment method and the amount of oxygen in the exhaust gas is smaller than that in the first air-fuel ratio enrichment method. Yes, when the amount of oxygen in the exhaust gas per unit time flowing into the NOx trap catalyst is small, the first air-fuel ratio enrichment method is selected and the unit time flowing into the NOx trap catalyst When the amount of oxygen in the exhaust gas or large, characterized in that it comprises, and the air-fuel ratio enrichment means for selecting the second air-fuel ratio enrichment methods.
[0008]
According to a second aspect of the present invention, there is provided target rich air-fuel ratio setting means for setting a target rich air-fuel ratio that makes the exhaust air-fuel ratio rich, and the air-fuel ratio enrichment means is configured to set the first rich air-fuel ratio when the exhaust air-fuel ratio is made rich. In both the air-fuel ratio enrichment method and the second air-fuel ratio enrichment method, the exhaust air-fuel ratio is set to the target rich air-fuel ratio.
According to a third aspect of the present invention, the air-fuel ratio enrichment means considers that the amount of oxygen in the exhaust per unit time flowing into the NOx trap catalyst increases when the space velocity of the exhaust is large.
[0009]
According to a fourth aspect of the present invention, the air-fuel ratio enriching unit includes a space velocity estimating unit that estimates the space velocity based on at least one of an intake air amount, an engine speed, and a fuel injection amount. And
According to a fifth aspect of the present invention, the engine includes an EGR valve disposed in an EGR passage that recirculates part of the exhaust gas from an engine exhaust passage to the intake passage, and an intake throttle valve disposed in the intake passage of the engine, When the first air-fuel ratio enrichment method is selected, the air-fuel ratio enrichment means enriches the exhaust air-fuel ratio at least with the EGR valve, and when the second air-fuel ratio enrichment method is selected, the intake throttle valve The exhaust air-fuel ratio is made rich.
According to a sixth aspect of the present invention, the air-fuel ratio enrichment means controls the intake throttle valve while continuing the EGR if the EGR is performed when the first air-fuel ratio enrichment method is selected. When the second air-fuel ratio enrichment method is selected when the intake air amount is controlled by controlling the intake air amount and the second air-fuel ratio enrichment method is selected, EGR is stopped and the intake throttle valve is controlled by controlling the intake throttle valve. The exhaust air / fuel ratio is stoichiometrically controlled by controlling the amount of intake air.
[0010]
According to a seventh aspect of the present invention, the air-fuel ratio enrichment means includes a fuel injection device that enables post injection that injects a small amount of fuel after main injection, and an intake throttle valve that is disposed in an intake passage of the engine. When the first air-fuel ratio enrichment method is selected, the exhaust air-fuel ratio is enriched by at least post injection, and when the second air-fuel ratio enrichment method is selected, the exhaust air-fuel ratio is selected by the intake throttle valve. It is characterized by enriching.
In the invention of claim 8, the air-fuel ratio enrichment means enriches the exhaust air-fuel ratio by post injection and selects the second air-fuel ratio enrichment method when the first air-fuel ratio enrichment method is selected. In this case, the amount of oxygen in the exhaust gas is reduced by reducing the intake air amount by reducing the intake throttle valve opening, thereby reducing the post-injection amount or post-injection as the excess air ratio decreases. It is characterized by stopping.
[0011]
【The invention's effect】
According to the first aspect of the present invention, when the amount of oxygen in the exhaust gas per unit time flowing into the NOx trap catalyst is large when performing reduction purification by the rich spike of the NOx trap catalyst, the reduction is performed by the residual oxygen in the exhaust gas. By enriching the exhaust air / fuel ratio with an air / fuel ratio enrichment method that can reduce the amount of oxygen in the exhaust gas, the agent (HC, CO) may be consumed and NOx may not be sufficiently reduced and purified. , NOx purification performance can be maintained.
[0012]
According to the invention of claim 2, in any of the air-fuel ratio enrichment methods, by making the exhaust air-fuel ratio the same target rich air-fuel ratio by the same setting means, fluctuations in the exhaust air-fuel ratio can be prevented, and NOx, HC It is possible to suppress fluctuations in emissions such as CO.
According to the invention of claim 3, as the amount of gas passing through the NOx trap catalyst increases, the reaction time between the reducing agent and oxygen or NOx tends to be short, and the reaction at the catalyst tends not to be expected sufficiently. When the space velocity is high, it is possible to control based on the space velocity by assuming that the amount of oxygen in the exhaust gas per unit time flowing into the NOx trap catalyst becomes large.
[0013]
According to the fourth aspect of the present invention, the space velocity can be controlled with the existing parameters by estimating the space velocity based on at least one of the intake air amount, the engine speed, and the fuel injection amount.
According to the fifth and sixth aspects of the invention, when the exhaust air-fuel ratio is enriched while performing EGR when performing a normal rich spike, the EGR is stopped when reducing the oxygen amount in the exhaust during the rich spike. Then, by enriching the exhaust air-fuel ratio with the intake throttle valve, it is possible to combust all the oxygen that should be consumed by combustion, and to reliably reduce the amount of oxygen in the exhaust.
[0014]
According to the inventions of claims 7 and 8 , when the exhaust air-fuel ratio is enriched by post-injection when normal rich spike is performed, post-injection is stopped when reducing the amount of oxygen in the exhaust at the time of rich spike Alternatively, the amount of oxygen in the exhaust gas can be reliably reduced by reducing and enriching the exhaust air / fuel ratio with the intake throttle valve.
[0015]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described below with reference to the drawings.
FIG. 1 is a system diagram of an internal combustion engine (here, a diesel engine) showing an embodiment of the present invention.
The intake passage 2 of the diesel engine 1 is provided with an intake compressor of a variable nozzle type turbocharger 3. The intake air is supercharged by the intake compressor, cooled by the intercooler 4, passed through the intake throttle valve 5, and then the collector 6 and then flows into the combustion chamber of each cylinder. The fuel is increased in pressure by the common rail type fuel injection device, that is, by the high pressure fuel pump 7, sent to the common rail 8, and directly injected from the fuel injection valve 9 of each cylinder into the combustion chamber. The air that has flowed into the combustion chamber and the injected fuel are combusted by compression ignition, and the exhaust gas flows out to the exhaust passage 10.
[0016]
Part of the exhaust gas flowing into the exhaust passage 10 is recirculated to the intake side via the EGR valve 12 through the EGR passage 11 as EGR gas. The remainder of the exhaust passes through the exhaust turbine of the variable nozzle type turbocharger 3 and drives it.
Here, downstream of the exhaust turbine in the exhaust passage 10, for exhaust purification, NOx in the exhaust is trapped when the air-fuel ratio of the inflowing exhaust is lean, and trapped when the air-fuel ratio of the inflowing exhaust is rich A NOx trap catalyst 13 for reducing and purifying NOx is disposed. Further, the NOx trap catalyst 13 carries a noble metal and has a function of oxidizing HC and CO in the exhaust gas, and serves as a NOx trap catalyst with an oxidation function.
[0017]
In order to control the engine 1, the control unit 20 includes a rotation speed sensor 21 for detecting the engine rotation speed Ne, an accelerator opening sensor 22 for detecting the accelerator opening APO, and an air flow meter 23 for detecting the intake air amount Qa. A signal is being input.
Further, a catalyst temperature sensor 24 for detecting the temperature (catalyst temperature) Tc of the NOx trap catalyst 13 and an air / fuel ratio sensor 25 for detecting the exhaust air / fuel ratio on the outlet side of the NOx trap catalyst 13 in the exhaust passage 10 are provided. A signal is also input to the control unit 20. However, the temperature of the NOx trap catalyst 13 may be detected from the exhaust temperature in the vicinity (especially the downstream side).
[0018]
Based on these input signals, the control unit 20 controls the fuel injection amount and the injection timing of the main injection by the fuel injection valve 9 and the post injection performed after the main injection (expansion stroke or exhaust stroke) under predetermined operating conditions. The fuel injection command signal to the fuel injection valve 9, the opening command signal to the intake throttle valve 5, the opening command signal to the EGR valve 12, etc. are output.
[0019]
Here, the control unit 20 performs exhaust purification control for reducing and purifying NOx trapped and accumulated in the NOx trap catalyst 13, and the exhaust purification control will be described in detail below.
2 to 4 are flowcharts of exhaust purification control executed by the control unit 20.
[0020]
In S1-1, the engine speed Ne, the accelerator opening APO, the intake air amount Qa, and the catalyst temperature Tc are detected based on signals from the rotation speed sensor, accelerator opening sensor, air flow meter, and catalyst temperature sensor.
In S1-2, the fuel injection amount Qf for main injection is calculated by referring to a map using the engine speed Ne and the accelerator opening APO as parameters.
[0021]
In S1-3, the NOx deposition amount trapped and deposited on the NOx trap catalyst is detected. However, since it is difficult to directly detect the NOx accumulation amount, the NOx generation amount per unit time is predicted from the engine speed Ne and the fuel injection amount Qf, and the NOx accumulation amount per unit time is considered in consideration of the trap rate. It is detected indirectly by calculating and accumulating these. Alternatively, the NOx accumulation amount may be estimated from the integrated value of the engine speed.
[0022]
In S1-4, it is determined whether or not the reg1 flag indicating that the rich spike mode is active when the catalyst is active is set. When the reg1 flag = 1, the control proceeds to the rich spike mode control when the catalyst is active after S2-1 (FIG. 3).
In S1-5, it is determined whether or not the reg2 flag indicating that the rich spike mode when the catalyst activity is low is set. When the reg2 flag = 1, the control proceeds to the rich spike mode control when the catalyst activity is low after S3-1 (FIG. 4).
[0023]
In S1-6, it is determined whether or not the NOx accumulation amount detected in S1-3 is greater than a predetermined value NOx1 in order to determine whether or not it is the NOx trap catalyst regeneration timing (reduction purification timing).
If the NOx accumulation amount is equal to or less than the predetermined value NOx1, it is not the regeneration time, so the process is terminated. If the NOx accumulation amount exceeds the predetermined value NOx1, it is determined as the regeneration time, and the process proceeds to S1-7.
[0024]
In S1-7, the activity of the NOx trap catalyst is determined. Here, as shown in FIG. 5, the NOx purification performance of the catalyst starts to appear from the light-off temperature T2, but is not yet sufficient. Therefore, the catalyst is activated when the temperature becomes higher than the temperature T1 at which the NOx purification rate becomes sufficiently high. Judge. It is determined that the activity is low between T1 and T2, and it is determined that there is no activity at a temperature lower than T2.
[0025]
Therefore, in S1-7, it is determined whether or not the catalyst temperature Tc exceeds T1, and when Tc> T1, it is determined in S1-8 that the catalyst is active, In order to enter the rich spike mode, the reg1 flag is set to 1.
In the case of Tc ≦ T1, it is determined in S1-9 whether or not the catalyst temperature Tc exceeds T2, that is, whether or not the catalyst temperature Tc is between T1 and T2, and if Tc> T2, In S1-10, it is determined that the activity is low, and the reg2 flag is set to 1 to enter the rich spike mode when the catalyst activity is low.
[0026]
In the case of Tc ≦ T2, since there is no activity and no purification performance can be expected, the process is terminated by waiting for the regeneration process until it is warmed up.
Next, the rich spike mode at the time of catalyst activation after S2-1 when the reg1 flag = 1 is described.
In S2-1, the space velocity SV at the exhaust catalyst is estimated. Here, the amount of intake air Qa is used, and a value obtained by dividing this by the catalyst capacity is defined as a space velocity SV. However, since the catalyst capacity is constant for each engine model, the intake air amount Qa may be used as it is as an index of space velocity. In addition to using the intake air amount Qa in this way, the engine speed Ne, the fuel injection amount Qf, and the like can also be used.
[0027]
Here, as shown in FIG. 6, in the state where the exhaust λ <1 (rich) and no oxygen, NOx can be purified by the NOx trap catalyst by reacting with the reducing components (HC, CO) in the exhaust. As shown in FIG. 7 or FIG. 8, when oxygen is present even if the exhaust λ <1 (rich), first, oxygen is consumed to create a reducing atmosphere, and NOx is purified in the reducing atmosphere. Become.
[0028]
FIGS. 7 and 8 both show the case where exhaust λ <1 (rich) and oxygen is present, but FIG. 7 shows a condition where the space velocity SV of the exhaust catalyst is small, and FIG. 8 shows a condition where the space velocity SV is large.
As shown in FIG. 7, when the space velocity SV is low, the reaction time is long. Therefore, even if oxygen remains in the exhaust gas, it is possible to consume oxygen and create a reducing atmosphere. The original function can be reduced. That is, when SV is small, it is possible to secure a reaction time for consuming oxygen to create a reducing atmosphere and purifying NOx trapped in the reducing atmosphere.
[0029]
On the other hand, as shown in FIG. 8, when the space velocity SV is large, the reaction time is short. Therefore, when oxygen remains in the exhaust gas, most of the time is consumed to consume oxygen. The time to become less. For this reason, NOx purification performance will fall.
In addition, as shown in FIG. 9, when rich operation is realized in a diesel engine, combustion is slightly unstable when EGR is performed, and the exhaust gas is exhausted with the same exhaust λ as compared to the condition where EGR is not performed. The amount of residual oxygen increases (the more oxygen, the more CO and HC emissions).
[0030]
Therefore, when the space velocity SV is large, the reaction time is shortened, so that most of the reaction time (catalyst passage time) is consumed to consume oxygen. In addition, if the amount of residual oxygen is large, the NOx purification rate decreases even if the exhaust λ <1 (rich), so it is necessary to reduce the amount of residual oxygen in the exhaust during a rich spike.
[0031]
As a method, for example, it is possible to reduce the amount of residual oxygen by not performing EGR under conditions where EGR is performed.
For this reason, control is performed as follows from S2-2.
In S2-2, it is determined whether the space velocity SV is greater than a predetermined value SV1.
When the space velocity SV ≦ SV1, the process proceeds to S2-3, and the first air-fuel ratio enrichment method for enriching the exhaust air-fuel ratio is selected to enrich the exhaust air-fuel ratio. Specifically, if the operating condition is to perform EGR, the target λ during rich spike is set to a predetermined value λ1 while continuing the EGR, and the target intake air amount shown in FIG. To achieve the target λ = λ1. As for the error, feedback control is performed based on the signal from the air-fuel ratio sensor on the catalyst outlet side.
[0032]
When the space velocity SV> SV1, the process proceeds to S2-4, and the second air-fuel ratio enrichment method is selected in which the amount of oxygen in the exhaust gas is made smaller than that in the first air-fuel ratio enrichment method and the exhaust air-fuel ratio is made rich. To enrich the exhaust air-fuel ratio. Specifically, if the operating condition is to perform EGR, EGR is stopped, the target λ at the time of rich spike is set to a predetermined value λ1, and the EGR shown in FIG. 11 is not performed by the control of the intake throttle valve To achieve the target λ = λ1. As for the error, feedback control is performed based on the signal from the air-fuel ratio sensor on the catalyst outlet side.
[0033]
In this case, by setting the target λ during rich spike to the same value as λ1 set in S2-3 even if the residual oxygen amount is reduced, NOx purification due to emission deterioration due to excessive spikes and shallow spikes A decrease in performance can be suppressed.
Supplementing the air-fuel ratio enrichment method, the normal rich spike, that is, the rich spike when there is no need to reduce the oxygen amount (first air-fuel ratio enrichment method) is at least one of EGR, intake throttle valve, and post injection. Use one.
[0034]
When using EGR, when performing a normal rich spike, the EGR valve opening is increased so as to increase the EGR ratio, and the intake throttle valve opening is decreased to reduce the excess air ratio.
By the way, since the distribution of the EGR gas sucked into the cylinder (oxygen concentration in the cylinder) is not uniform, when the injected fuel burns, the fuel that exists in the region where the EGR gas concentration is high (low oxygen concentration) Is difficult to burn and is discharged as HC. That is, since the fuel to be combusted does not combust, the oxygen to be consumed by the combustion is discharged as it is without being consumed.
[0035]
Therefore, when it is desired to reduce the amount of oxygen during the rich spike (in the second air-fuel ratio enrichment method), by realizing the target excess air ratio without EGR, all the oxygen that should be consumed by combustion is burned. As a result, the amount of oxygen in the exhaust gas can be reduced.
That is, when the rich spike when there is no need to reduce the oxygen amount is performed by the EGR and the intake throttle, the rich spike when reducing the oxygen amount is performed only by the intake throttle (no EGR).
[0036]
Further, when the rich spike when there is no need to reduce the oxygen amount is performed only by the post injection (the characteristic diagram of the post injection amount for the rich spike is shown in FIG. 12), the rich spike when the oxygen amount is reduced. This can be realized by reducing the intake air amount by reducing the intake throttle valve opening to reduce the amount of brew in the exhaust, and thereby reducing the post injection amount by the amount that the excess air ratio decreases.
[0037]
That is, when controlling the exhaust λ by controlling the injection amount such as post injection (by increasing the unburned fuel), the exhaust λ control by controlling the air amount is changed from the exhaust λ control by controlling the unburned fuel. By switching to, it becomes possible to reduce the amount of residual oxygen in the exhaust.
Furthermore, if the rich spike when there is no need to reduce the oxygen amount is performed by EGR, intake throttle and post injection, is the rich spike when reducing the oxygen amount only the intake throttle (no EGR, no post injection)? Alternatively, it can be performed by reducing the intake throttle and the post injection amount (the reduction amount depends on the intake throttle amount).
[0038]
After the rich spike starts, the process proceeds to S2-5.
In S2-5, it is determined whether or not the elapsed time (rich spike time) t from the start of the rich spike has exceeded a predetermined time t1, and if so, it is considered that the regeneration of the catalyst has been completed. The accumulated value of the NOx accumulation amount is cleared at 6 and the reg1 flag is set to 0 at S2-7.
[0039]
Next, the rich spike mode when the catalyst activity after S3-1 when the reg2 flag is 1 is low will be described. When the catalytic activity is low, an exceptional measure is taken because the NOx purification rate itself of the NOx trap catalyst is lowered. That is, when the active state is low and the NOx purification rate of the NOx trap catalyst is low, when the SV is large, a sufficient NOx purification performance cannot be obtained even if the second air-fuel ratio enrichment method is selected, so a rich spike is forgotten. On the other hand, when SV is small, the remaining oxygen in the exhaust gas is smaller than the first air-fuel ratio enrichment means in order to compensate for the low NOx purification performance by increasing the amount of reducing agents (HC, CO) that react with NOx. Select the fuel enrichment method. As a result, it is possible to suppress a decrease in NOx purification performance due to a decrease in the NOx purification rate.
[0040]
In S3-1, as in S2-1, the space velocity SV at the exhaust catalyst is estimated.
In S3-2, it is determined whether or not the space velocity SV is greater than a predetermined value SV1.
When the space velocity SV ≦ SV1, the process proceeds to S3-3, and the second air-fuel ratio enrichment method is selected in which the amount of oxygen in the exhaust gas is made smaller than that in the first air-fuel ratio enrichment method and the exhaust air-fuel ratio is made rich. To enrich the exhaust air-fuel ratio. Specifically, if the operating condition is to perform EGR, EGR is stopped, the target λ at the time of rich spike is set to a predetermined value λ1, and the EGR shown in FIG. 11 is not performed by controlling the intake throttle valve To achieve the target λ = λ1. As for the error, feedback control is performed based on the signal from the air-fuel ratio sensor on the catalyst outlet side. Also in this case, the oxygen amount in the exhaust gas may be reduced by the other method described above.
[0041]
In S3-4, it is determined whether or not the elapsed time (rich spike time) t from the start of the rich spike has exceeded a predetermined time t1, and if so, it is considered that the regeneration of the catalyst has been completed. The accumulated value of the NOx accumulation amount is cleared at 5 and the reg2 flag is set to 0 at S3-6.
On the other hand, if SV> SV1 in the determination in S3-2, since the catalyst activity is low, sufficient NOx purification performance cannot be obtained even if the spike is performed. In step 6, the reg2 flag is set to 0 and the process is terminated.
[0042]
In this embodiment, the portion S1-6 in FIG. 2 corresponds to the reduction purification timing determination means, the portion S2-1 in FIG. 3 corresponds to the space velocity estimation means, and S2-2 to S2-2 in FIG. S2-4 corresponds to air-fuel ratio enrichment means including target rich air-fuel ratio setting means.
[Brief description of the drawings]
FIG. 1 is a system diagram of an engine showing an embodiment of the present invention. FIG. 2 is a flowchart of exhaust purification control (part 1).
FIG. 3 is a flowchart of exhaust purification control (part 2).
FIG. 4 is a flowchart of exhaust purification control (part 3).
FIG. 5 is a diagram showing the relationship between temperature and catalytic activity. FIG. 6 is a diagram showing a reaction in the absence of oxygen at exhaust λ <1. FIG. 7 is a state where oxygen is present at exhaust λ <1 and SV is small. FIG. 8 is a diagram showing a reaction in a state where exhaust λ <1 and oxygen is present and SV is large. FIG. 9 is a diagram showing a change in components in exhaust with and without EGR. FIG. 10 is a rich spike. FIG. 11 is a diagram showing a target intake air amount at the time of rich spike without EGR. FIG. 12 is a diagram showing a post injection amount at the time of rich spike.
DESCRIPTION OF SYMBOLS 1 Engine 2 Intake passage 5 Intake throttle valve 8 Common rail 9 Fuel injection valve 10 Exhaust passage 11 EGR passage 12 EGR valve 13 NOx trap catalyst 20 Control unit 21 Rotational speed sensor 22 Acceleration opening degree sensor 23 Air flow meter 24 Catalyst temperature sensor 25 Air-fuel ratio Sensor

Claims (8)

An NOx trap catalyst that is disposed in the exhaust passage of the engine and traps NOx in the exhaust when the air-fuel ratio of the inflowing exhaust is lean and reduces and purifies NOx trapped when the air-fuel ratio of the inflowing exhaust is rich;
Reduction purification timing determination means for determining the reduction purification timing of the NOx trap catalyst;
The first air-fuel ratio enrichment method for enriching the exhaust air-fuel ratio and the exhaust air-fuel ratio at the same value as the first air-fuel ratio enrichment method and the amount of oxygen in the exhaust are reduced to the first air-fuel ratio at the reduction purification timing. ratio is a second air-fuel ratio enrichment method of less than enrichment methods can selectively switching, the first air-fuel ratio rich when the oxygen content in the exhaust gas per unit time flowing into the NOx trap catalyst is smaller An air-fuel ratio enrichment means that selects the second air-fuel ratio enrichment method when the oxygen amount in the exhaust gas per unit time flowing into the NOx trap catalyst is large,
An exhaust emission control device for an internal combustion engine, comprising: A target rich air-fuel ratio setting means for setting a target rich air-fuel ratio that makes the exhaust air-fuel ratio rich;
When the exhaust air-fuel ratio is made rich, the air-fuel ratio enriching means makes the exhaust air-fuel ratio the target rich air-fuel ratio in both the first air-fuel ratio enrichment method and the second air-fuel ratio enrichment method. The exhaust emission control device for an internal combustion engine according to claim 1.   3. The air-fuel ratio enrichment means considers that the amount of oxygen in the exhaust per unit time flowing into the NOx trap catalyst increases when the space velocity of the exhaust is large. An exhaust purification device for an internal combustion engine. The air-fuel ratio enrichment means, intake air amount, based on at least one of the engine speed and the fuel injection amount according to claim 3, characterized in that it comprises a space velocity estimating means for estimating the spatial velocity An exhaust purification device for an internal combustion engine. An EGR valve disposed in an EGR passage that recirculates part of the exhaust from the exhaust passage of the engine to the intake passage, and an intake throttle valve disposed in the intake passage of the engine,
The air-fuel ratio enrichment means enriches the exhaust air-fuel ratio with at least an EGR valve when the first air-fuel ratio enrichment method is selected, and selects the intake air when the second air-fuel ratio enrichment method is selected. The exhaust gas purification apparatus for an internal combustion engine according to any one of claims 1 to 4, wherein the exhaust air-fuel ratio is enriched by a throttle valve.   The air-fuel ratio enrichment means controls the intake air amount by controlling the intake throttle valve while continuing the EGR if the condition for performing the EGR is selected when the first air-fuel ratio enrichment method is selected. When the exhaust air-fuel ratio is controlled to stoichiometric and the second air-fuel ratio enrichment method is selected, if the EGR is performed, EGR is stopped and the intake air amount is controlled by controlling the intake throttle valve. 6. The exhaust emission control device for an internal combustion engine according to claim 5, wherein the exhaust air-fuel ratio is controlled to stoichiometric. A fuel injection device that enables post injection that injects a small amount of fuel after the main injection, and an intake throttle valve that is arranged in the intake passage of the engine,
The air-fuel ratio enrichment means enriches the exhaust air-fuel ratio by at least post injection when the first air-fuel ratio enrichment method is selected, and selects the intake air when the second air-fuel ratio enrichment method is selected. The exhaust gas purification apparatus for an internal combustion engine according to any one of claims 1 to 4, wherein the exhaust air-fuel ratio is enriched by a throttle valve.   The air-fuel ratio enriching means enriches the exhaust air-fuel ratio by post injection when the first air-fuel ratio enrichment method is selected, and selects the intake air throttle when the second air-fuel ratio enrichment method is selected. The amount of oxygen in the exhaust gas is reduced by reducing the valve opening and reducing the amount of intake air, thereby reducing the post-injection amount or stopping post-injection as the excess air ratio decreases. The exhaust emission control device for an internal combustion engine according to claim 7.

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